The present invention relates generally to hydraulically actuated power steering systems utilizing rotary control valves and, more particularly, to an apparatus and method for improving system stability.
Power steering systems are used in motor vehicles to augment the steering effort applied to the steering wheel. Many conventional power steering systems employ an open-center rotary control valve (hereinafter "rotary valve") having "follow along" position feedback. In rotary valve equipped systems, "road feel" is artificially induced by deflection of a rotationally compliant torsion bar.
In general, power steering systems can be described as closed-loop servo systems wherein input signals are received from the motor vehicle upon the vehicle operator applying a steering torque to the steering wheel. Output signals are compared therewith via the above noted "follow along" position feedback. Like all functional closed-loop servo systems, power steering systems typically have open loop gain with an absolute value that is greater than 1.0 over much of their operational frequency range. Ideally, stable system operation requires unity gain cross-over with a phase lag angle of less than 180 degrees at some higher frequency. (A thorough discussion of servo system stability is contained in a book entitled FEEDBACK AND CONTROL SYSTEMS by Di Stefano III, Stubberud and Williams, and published by the McGraw-Hill Book Company as one of Schaum's Outline Series.)
Unfortunately, many conventional rotary valve equipped power steering systems do not achieve unity gain cross-over with a phase lag angle of less than 180 degrees under all operating conditions. Instead, these systems rely on Coulomb friction to attenuate resulting system oscillation. This is generally implemented with an O-ring "brake" which disrupts normal system roll-off characteristics in order to preclude system oscillation. An exemplary rotary valve constructed to include O-ring "brake" is illustrated in FIG. 2 of U.S. Pat. No. 4,452,274, entitled ROTARY VALVE FOR POWER STEERING SYSTEM, issued June 5, 1984 to Haga et al, the specification and drawings of which are expressly incorporated by reference herein. Although it is not specifically called out by reference number, the O-ring is located in a groove in steering shaft 24 to the left of port 38 in FIG. 2 of U.S. Pat. No. 4,452,274. Such O-ring mounting grooves are typically ported to incoming hydraulic fluid pressure such that the O-ring acts as a brake between the steering shaft and the valve sleeve whenever the incoming hydraulic fluid pressure is high.
Alternatively, relatively stable system operation for rotary valve equipped power steering systems has been obtained by "detuning" the system using an abnormally slow bypass flow regulation response (of their pumping systems). In effect, the slow bypass flow regulation response acts as a dominant pole for roll off open-loop gain. Such "detuning" provides a unity gain cross-over with a phase lag angle of less than 180 degrees which allows the O-ring "brake" to be eliminated.
However, the stable system operation requires that the slow response must be effective at high pressures despite the fact that bypass flow regulation response time characteristically increases with a reduction in fluid pressure. For these reasons, flow regulation response times can potentially exceed one second at low steering forces. This results in steering assist response times that are so slow that the vehicle operator may not perceive any steering assist at the low steering forces.
Accordingly, the present invention describes various embodiments of an improved rotary valve equipped power steering system which overcome the disadvantages of the prior art. More particularly, various methods of achieving "stable" unity gain cross-over are described. In a first preferred embodiment, the steering wheel is coupled to the rotary control valve via a coupling device having a rotational stiffness characteristic that is less than the rotational stiffness associated with the rotary valve itself. This results in a general decrease in the open loop gain whereby unity gain cross-over is achieved at a lower frequency with a reduced phase lag angle.
In a second preferred embodiment, the coupling device is modified to include a damping media in addition to the above noted reduced rotational stiffness. The damping media adds a zero function in the open loop gain which causes unity gain cross-over to be achieved with less phase lag angle.
In a third preferred embodiment, the power steering systems described with reference to the first and second embodiments are modified to include, constant flow rate pumping systems. Since fluid is delivered to the rotary valve at a substantially constant flow rate, substantially instantaneous pressure response is achieved. As a result, the above noted bypass flow regulation response pole is virtually eliminated whereby unity gain crossover occurs with still less phase lag angle.
Various other objects and advantages of the present invention will become more apparent to one skilled in the art from reading the following specification taken in conjunction with the appended claims and the following drawings.